Re: [PATCH 1/6] sched/proxy: Remove superfluous clear_task_blocked_in()

[K Prateek Nayak]

Hello John,

On 5/29/2026 4:50 AM, John Stultz wrote:
> However, even with the fix I poined out, I've unfortunately hit races
> with the ww_mutex selftest at the point of this patch in the series.
> Basically between commit
>   1b89b7b21bf5 ("sched/proxy: Remove superfluous clear_task_blocked_in()")
> and
>   a8be1edac5a1 ("sched/proxy: Remove PROXY_WAKING")
> 
> I'm currently tracing down exactly why the race is cropping up but I
> believe the chunk removed in this case is avoiding cases where we end
> up getting PROXY_WAKING set on a TASK_RUNNING task.

This seems to be the failure path:

  /* Task p*/
  mutex_lock(mutex)
    ...                            try_to_wake_up(p)
    schedule_preempt_disabled()      ttwu_runnable()
      __schedule()                     __task_rq_lock() /* Wins */
        rq_lock() /* Waits */            if (task_on_rq_queued(p))
                                           /*
                                            * p->is_blocked is still not set!
                                            * proxy_needs_return() bails out early.
                                            */
                                           ttwu_do_wakeup()
                                             p->__state = TASK_RUNNING;
                                         __tsk_rq_unlock();
        ...
        /* p->__state = TASK_RUNNING */
        prev_state = p->__state;
        if (prev_state && ...) {
           /*
            * Skipped since task is
            * already TASK_RUNNING
            */
        }

        /* p->is_blocked = 0; p->blocked_on = PROXY_WAKING */
        next = p;

      /* Returns from schedule_preempt_disabled()
      set_task_blocked_on(p, mutex)

         !!! p->blocked_on == PROXY_WAKING && p->blocked_on != mutex !!!
---

Also proxy_needs_return() bails out too early - a wakeup from signal
should still clear p->blocked_on even if p->wake_cpu is same as
task_cpu().


I think we need the following at commit 83f9b04ef50c ("sched/proxy:
Switch proxy to use p->is_blocked") to clear p->blocked_on in the wakeup
path irrespective of p->is_blocked:

  (Lightly tested)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index a125e65c35bb..fe903976fd09 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3764,28 +3764,28 @@ static inline void proxy_reset_donor(struct rq *rq)
  */
 static inline bool proxy_needs_return(struct rq *rq, struct task_struct *p)
 {
-	if (!p->is_blocked)
-		return false;
-
-	/*
-	 * Typically per __set_task_cpu(), task_cpu(p) == p->wake_cpu.
-	 *
-	 * However, proxy_set_task_cpu() is such that it preserves the
-	 * original cpu in p->wake_cpu while migrating p for proxy reasons
-	 * (possibly outside of the allowed p->cpus_ptr).
-	 *
-	 * Furthermore, migration_cpu_stop() / __migrate_swap_task(), will
-	 * only set p->wake_cpu when !p->on_rq, and since here p->on_rq, this
-	 * will not apply. But if it did, this check is the safe way around
-	 * and would migrate.
-	 */
-	if (task_cpu(p) == p->wake_cpu)
+	if (!task_is_blocked(p))
 		return false;
 
 	scoped_guard(raw_spinlock, &p->blocked_lock) {
 		/* Task is waking up; clear any blocked_on relationship */
 		__clear_task_blocked_on(p, NULL);
 
+		/*
+		 * Typically per __set_task_cpu(), task_cpu(p) == p->wake_cpu.
+		 *
+		 * However, proxy_set_task_cpu() is such that it preserves the
+		 * original cpu in p->wake_cpu while migrating p for proxy reasons
+		 * (possibly outside of the allowed p->cpus_ptr).
+		 *
+		 * Furthermore, migration_cpu_stop() / __migrate_swap_task(), will
+		 * only set p->wake_cpu when !p->on_rq, and since here p->on_rq, this
+		 * will not apply. But if it did, this check is the safe way around
+		 * and would migrate.
+		 */
+		if (task_cpu(p) == p->wake_cpu)
+			return false;
+
 		/* If already current, don't need to return migrate */
 		if (task_current(rq, p))
 			return false;
---

Part of that belongs in commit e2ff8b7bde07 ("sched/proxy: Only return
migrate when needed") and the first hunk of 83f9b04ef50c ("sched/proxy:
Switch proxy to use p->is_blocked") in proxy_needs_return() should be
dropped.

-- 
Thanks and Regards,
Prateek